Improving TDR Measurements through Accounting for Soil Shrinkage Properties

Abstract

Time domain reflectometry (TDR) is a widely used tool for indirect measurement of soil moisture content. Empirical formulation is used to link the apparent dielectric constant of soil to the volumetric moisture content (Topp’s Equation: Topp et al., 1980) or gravimetric moisture content as a function of soil bulk density (Siddiqui and Drnevich Equation: Siddiqui and Drnevich, 1995). This paper introduces a methodology to account for soil volume change by integrating the true bulk density of the soil into the measurements using the soil Shrinkage Curve (specific volume (cm3/g) versus gravimetric water content). Thus, bulk density becomes a non-constant parameter that can be calculated as a function of the soil water content in the Siddiqui and Drnevich Equation. Experimental evidence demonstrates accounting for soil shrinkage improves the accuracy of TDR measured moisture contents and allows for estimating the shrinkage curve. Direct water content calculation for the Chalmers soil was compared to water contents from TDR readings with and without shrinkage corrections; those with shrinkage corrections showed significantly improved accuracy in TDR-determined soil moisture.